OBSERVATIONS of developmental events such as the sorting out of cells during organogenesis suggest that highly specific mechanisms exist for cells to migrate apart or aggregate together. Several assays of cell-to-cell binding using cells from developing tissues have provided probable in vitro analogues of these recognition mechanisms. When cells from various organs of the embryo are dissociated, re-aggregation occurs in a tissue-specific manner with, for example, liver cells binding preferentially to other liver cells and brain cells binding preferentially to other brain cells. A number of groups using different assay systems have agreed with this general finding1–4. Furthermore, our own work with chick embryonic neural retina cells has shown that proteolytic enzyme treatment of these cells removes structures which are necessary for specific cell-cell adhesion to occur5. The relevant ‘recognition’ moieties on the cell surface may be glycoproteins6–8. We have investigated cell–cell adhesion using embryonic tissues from two strains of mice, C57B1/10 and C3H, which differ from one another at the H–2 locus as well as at a number of minor H–loci.